1. Field of the Invention
The present invention relates to structures for thermal management of planar solid oxide fuel cell systems, more particularly, those that enable internal reforming for thermal management at the device or cell level.
2. Description of the Related Art
The SOFC is widely considered the most desirable fuel cell for generating electricity from hydrocarbon fuels because it is highly efficient, and can at least partially internally reform hydrocarbon fuels. Solid Oxide Fuel Cells (SOFC) are reaching commercialization with several hundreds of residential stationary power units (about 1 kW) being tested in Europe and larger units (250 kW or above) being evaluated by various utility companies world wide. SOFC also are emerging as a possible power option for hybrid electrical vehicles and in auxiliary power units. Due to their high energy conversion efficiency (up to 40-60%), low toxic emissions, and flexibility in fuel choice (natural gas, diesel, gasoline, liquid petroleum, alcohol, coal gas, etc.), SOFC are being developed for the whole range of possible applications: stationary, transportation (or mobile), military, and portable.
Solid oxide fuel cells (SOFC) include a solid electrolyte medium separating the electrodes, typically a ceramic material such as yttria-stabilized zirconia (YSZ). The anode is usually a porous metal composite, such as Ni-cermet, and the cathode typically lanthanum-strontium manganese oxide (LSM). In SOFCs, hydrogen (or raw fuel) is converted to water at the anode while oxygen is converted into oxygen ions that migrate through the electrolyte ceramic to the anode. As the main current carriers are oxygen ions, which diffuse slowly at room temperature, operating temperatures ranging from about 800° C. to 1000° C. are typically used. These high temperatures require exotic and costly materials such as lanthanum chromite to be used for current collectors (interconnects).
FIG. 12 shows a conventional fuel cell assembly, which includes four basic elements: an anode, a cathode, an electrolyte, and an interconnect. The oxidant, usually air, is fed through a plurality of channels defined by the cathode and grooves in the interconnect to provide needed oxygen for the dominant reaction of hydrogen oxidation. In addition, the oxidant serves as a coolant when the fuel cell is hydrogen fed, and provides heat needed when the fuel cell assembly is methane fed (i.e. cells including endothermic internal methane reforming). In a manner complementary to the structure for providing oxidant to a cell, fuel is fed through a plurality of channels on the anode side of the cell, which are defined by the grooves formed in the interconnect and the anode. With such a structure, oxygen ions are conducted by the electrolyte and electrons are conducted by the interconnect during the process of hydrogen oxidation.
The electricity providing chemical reaction occurring at elevated temperatures in the SOFC is hydrogen oxidation (H2+0.5O2→H2O), which is exothermic (ΔH=−241.8 kJ/mol−H2) in nature, and thus heats the reaction sites. Left unchecked, the cumulative temperature within a cell/stack can quickly exceed maximum material design temperatures. In addition, the mass flow rates and partial pressures of the reactants as well as gas-phase temperatures vary along the direction of fuel flow causing temperature gradients. Large temperature gradients result in thermal stresses in the fuel cell, which can cause damage to the fuel cell's components. As such, temperatures must be managed to within a target range. Conventional methods of managing temperature include use of variable oxidant and fuel flow rates, heat exchangers within stack structures, and internal reforming.
Some level of heat transfer from the reaction sites to the fuel and oxidant gases occurs during normal operation of a SOFC. The amount of thermal energy in the gas is from a combination of gas-phase reactions as well as exchange (heat transfer) with solid-phase parts of the cell (anode, cathode, electrolyte, bipolar plates, etc.). With only the oxidation reaction predominant, and neglecting solid-gas heat transfer, the primary method of managing heat is through the heat capacity of the gas according to the general heat exchange relationship:Q=q/CpρΔT  (1)where Q=volumetric flow rate (m3/s), q=amount of heat transferred (kJ/s), Cp=specific heat of gas (kJ/kg−K), ρ=density of gas (kg/m3), and ΔT=temperature differential between gas inlet and gas outlet (° K). From equation (1), it can be seen that the volumetric flow rate of the gas is inversely proportional to the change in temperature. Thus, a greater flow rate leads to a lower change in gas temperature. In this way, stack temperature can be controlled.
In some cases, however, it would not be practical or cost effective to use fuel and/or air flow rates alone to control temperature. Thus, alternative methods of heat exchange and/or recovery are needed. However the alternative methods of heat exchange and/or recovery add cost, size and complexity to SOFC design.
Another approach currently being considered involves introducing a portion of unreformed fuel such as methane into the fuel cell channels with some amount of water to perform steam methane reforming, which is highly endothermic. This method can reduce the system complexity (no need for external reformers) as well as manage temperature within and throughout the stack. Mathematical modeling of distributed feed profiles has been described in the literature. For instance, Ayman M., Al-Qattan, Donald J. Chmielewski, “Distributed Feed Design for SOFC's with Internal Reforming”, Journal of The Electrochemical Society, 151 (11) A1891-A1898 (2004), describes using mathematical modeling and simulation to analyze chemical and thermal gradients, predict thermal-chemical performance, and propose distributed feed concentrations. While the Ayman et al. publication describes the concept of distributed feed to manage thermal-chemical gradients, it does not evolve the distributed feed designs/results into potential physical embodiments. Furthermore, it does not define SOFC stack level thermal management concepts as described herein with respect to the present invention. (A stack is constructed of a plurality, usually more than three, stacked SOFC devices.)
Another conventional method for thermal management of a fuel cell described in PCT Application No. WO 03098728 to Foger et al. involves processing a fuel supply stream of hydrogen, steam, at least one carbon oxide, and optionally methane, using a methanator to produce a fuel cell supply stream comprising a controlled concentration of methane, and reforming within the fuel cell methane present in the fuel cell supply stream. The methanator adjusts in response to fluctuations in the temperature of the fuel cell such that the concentration of methane in the fuel cell supply stream is controlled in order to achieve a desired level of reforming of methane within the fuel cell. While the Foger et al. method and design provides temperature control of the planar SOFC stack via internal reforming, thermal management is of the complete stack without regard to individual devices, cells, or active area. Furthermore, temperature gradients in the cell/device can still exist along the fuel/oxidant flow direction and or along the direction of voltage build in the stack.
Thus, there remains a need in the art for more efficient, flexible and less complex structures and ways for managing fuel cell stack temperatures at the fuel cell or the fuel cell device level.